This invention relates to electroscopic solid toners that are useful as a binder in development powders for developing a latent electrostatic image produced by photoelectrostatic copy techniques. More particularly, it relates to electroscopic solid toners which are formulated with thermoplastic resins based on acrylics.
Electrostatic copying process are well known in which a photoconductive medium is charged by exposure to light passing through an original to produce an electrostatic charge which may be developed with an electroscopic development powder. A wide variety of photoconductive media may be employed, such as inorganic photoconductive materials, organic photoconductors and elemental photoconductors.
A wide variety of techniques are known for developing the charged photoconductive medium such as magnetic brush, powder cloud, liquid developers and cascading techniques.
The formulation of electroscopic development powders that are compatible in a particular photoelectrostatic copying environment has been widely sought in this art and is well developed. Most commercially available electroscopic development powders consist of two or more thermoplastic resins whose combination have the right triboelectric charge as well as a sufficiently low softening point for proper fusing. Difficulty has been encountered however, since the use of different types of resins in solid toners lead to some undesirable properties such as caking, humidity sensitivity, poor process control and variations in triboelectric charge with usage. Additionally, not all fusing techniques are the same. In the radiant fusing technique, particular difficulties arise because there is no direct contact for the transfer of heat to the copy as in the use of heated or pressure fusing rollers.
It has been found that the above mentioned difficulties encountered in the radiant fusing technique can be overcome by choosing a resin that will yield a toner with the right triboelectric properties and fusing characteristics. The resin should have a melting point of 75.degree.-100.degree.C, a melt index of 20-30, a dielectric constant of 4-5 and a good tendency to wet and penetrate paper. The resin should also be compatible with the normally used coloring agents.
The melt index, as used herein, is understood to be a measure of how many grams of the development powder will flow in 10 minutes at a temperature of 150.degree.C, which measurement is in accordance with ASTM 1238. The values for dielectric constant, conductance and dissipation factor included herein were measured in accordance with ASTM D-150-70. The melting points were measured using a Fisher Johns melting apparatus.